This invention relates generally to nuclear magnetic resonance spectrometers, and, more particular, to probeheads for nuclear magnetic resonance spectrometers.
In nuclear magnetic resonance spectrometers, it is generally known to place a probehead into a strong and homogeneous constant magnetic field. The probehead consists of one or more high-frequency coils generating a high-frequency magnetic field and irradiating same on a sample being enclosed by the probehead. Within the sample region, the high-frequency magnetic field is oriented perpendicularly to the homogeneous constant magnetic field.
U.S. Pat. No. 4,851,780 describes a probehead for a nuclear magnetic resonance spectrometer, comprising two high-frequency coils being switched in parallel and being fed simultaneously from a common high-frequency transmitter/receiver. The high-frequency coils are designed as saddle-coils and are arranged adjacent to each other along a common axis. The coils are fed via electrical lines being arranged between coil terminals and a high-frequency terminal on the one hand and a ground terminal, respectively, on the other hand, both electrical lines comprising matching capacitors.
However, as is generally known, saddle-coils can only be used in a symmetric arrangement, i.e. only an even number of coils may be used as a probehead coil structure. Moreover, when two saddle-coils are used in symmetric arrangement as a high-frequency coil in a nuclear magnetic resonance probehead, and a sample is inserted along the symmetric center line extending perpendicularly to the afore-mentioned axis, the sample is located at the maximum of high-frequency electric field which gives rise to dielectric losses and, hence, decreases the probehead quality factor.
In a university diploma thesis entitled "Multipuls-Dernresonanz-Spektroskopie in Festkorpern bei 270 MHz" of Post, H., as being available to the public at Max-Planck-Institut fur medizinische Forschung, Heidelberg, Federal Republic of Germany, 1976, a particular probehead is described that can be used in a nuclear magnetic resonance spectrometer having a super-conducting magnet. The probehead comprises a coil structure for irradiating the high-frequency magnetic field, the coil structure consisting of two almost identical coils being wound in opposite winding senses. The high-frequency voltage is fed to a central connection point of the coils whereas the two outer coil terminals are electrically connected to a common point.
Considering these prior art probeheads, the high-frequency magnetic field homogeneity may be increased, because the prior art probeheads having two oppositely wound sub-coils allow to use a winding number being four times as high as that of prior art coil structures having an integral spiral-shaped coil and having the same value of a series capacitor within a probehead tank circuit.
In an article published in "Journal of Magnetic Resonance", 59, pages 307 through 317 (1984), another probehead for a nuclear magnetic resonance spectrometer is described which comprises several sub-coils for a coil structure generating the high-frequency magnetic field. According to this article, the sub-coils may either be wound in the same winding sense or in opposite winding senses. In any case, the sub-coils are switched in parallel. Moreover, the article suggests to use more than two sub-coils arranged in parallel without, however, giving further details thereon.
U.S. Pat. No. 4,633,181 described a conventional integrally wound coil within a nuclear magnetic resonance spectrometer probehead, and having an adjustable capacitor in parallel to the coil. Moreover, two electric lines being connected to the coil terminals are each provided with further capacitors. In such a way, one can select higher capacitance values for each of the single capacitors. Therefore, the selected capacitors may better be realized in practice, because higher capacitance values distinguish more over ambient strayfield capacitances.
A similar arrangement is also known form an article of "Journal of Magnetic Resonance", 36, pages 447 through 451 (1979) in connection with nuclear magnetic resonance spectrometer probeheads.
However, as mentioned above, all of these prior art coil structures as used for nuclear magnetic resonance spectrometer probeheads have the common disadvantage that either relatively low capacitance values capacitors are used or we have an inhomogeneous high-frequency magnetic field, or one has the sample arranged at a point with maximum high-frequency electrical field and, therefore, considerable dielectric losses.
There is, therefore, a need for an improved probehead of this kind that avoids the drawbacks, explained before, without, however, giving up the advantages of prior art probehead structures. The present invention fulfills this need, and further provides related advantages.